As we move toward battery and hydrogen cell breakthroughs that could wean us off our addiction to oil, here's at least one engine design from yesteryear that ought to be examined a bit more. The free-piston engine, first invented in 1920, is cheap to build and roughly twice as efficient as current gas engines. Unlike its conventional counterpart, the free-piston engine doesn't have a mechanical connect between the piston and a crankshaft. Instead, magnets at the center of the piston's rod move past metal coils to create an electrical current. The engine's configuration allows it to combust fuel quicker, improving efficiency, emissions and easily optimized for different fuels. The bad news: they're hard to control (variations in combustion cycles can cause poor performance) and they're incredibly loud (the quick explosions need to be muffled somehow). But automakers such as GM and Volvo are already investigating putting the engines in future vehicles. Lets hope their forays into this don't take as long as their exploration of fuel cell options. [Technology Review via Treehugger]